Dr. Lekutat's answer:

For a long time it was believed that accumulation of lactic acid was the source of the unpleasant soreness in muscles that can appear after exercise. Nowadays scientists assume that tiny injuries to the muscle fibers are responsible for the aches and pains. The inflammation that occurs because of these microscopic tears in the tissue leads to the well-known soreness. If you remain slightly in motion, the inflammatory substances are actually eliminated from the muscle tissue more rapidly and the pain subsides. It is, however, important to continue exercising only gently and carefully. Putting too much strain on the muscles can further injure them and worsen the soreness.